As a conventional method of forming a surface protective sheet on the surface of a molded particle, insert molding is used in which a surface protective sheet including a hard coating layer formed on one side of a base sheet and an adhesive layer formed on the other side thereof is inserted into a mold and a resin is injected into an empty space in the mold and cooled, thereby producing a resin molded article and simultaneously attaching the surface protective sheet to the surface of the molded article. Generally, a thermosetting resin or active energy ray-curable resin is used as the resin to form the hard coating layer of the surface protective sheet.
In manufacture of a surface protective sheet, when a thermosetting resin is used for a hard coating layer and is cured by heating, a molded article generally has low chemical resistance and abrasion resistance on the surface thereof. On the other hand, when an active energy ray-curable resin is used for a hard coating layer and is cured, crosslinking density of the resin increases, thereby improving chemical resistance and abrasion resistance. However, cracking occurs at a portion of the hard coating layer disposed on a curved part of the molded article in bonding.
When UV curable acrylic hard coating materials, such as polyester, acrylate, urethane acrylate, and epoxy acrylate, which are organic materials, are used alone to form a hard coating film, the hard coating film does not have sufficient surface protective properties. Thus, various organic-inorganic coating solutions containing an inorganic material, e.g., colloidal silica particles, have been used for a hard coating film to improve hardness and abrasion resistance.
However, such a hard coating film suffers precipitation of colloidal silica particles modified with organic materials or silane on the surface thereon and still has unsatisfactory properties in terms of stability related to generation of gel, curing degree, optical transparency, and adhesion.
Further, a hard coating composition for a conventional in-mold decorative film requires flexibility for curvature-molding of injection molding, whereas the surface of a final product needs higher film strength. However, a hard coating layer will suffer from a loss of strength in order to satisfy such opposite properties.
Thus, there is a need for development of a hard coating composition having not only high elongation but also excellent strength of a coating film.